1. Field of the Invention
The present invention relates to an apparatus and method for driving a light emitting panel using capacitive light emitting elements such as organic electroluminescence elements.
2. Description of the Related Background Art
In recent years, with the trend of increasing the size of display devices, thinner display devices have been required, and a variety of thin display devices have been brought into practical use. An electroluminescence display composed of a plurality of organic electroluminescence elements arranged in a matrix has drawn attention as one of the thin display devices.
The organic electroluminescence element (hereinafter simply called the “EL element”) may be electrically represented as an equivalent circuit as illustrated in FIG. 1. As can be seen from FIG. 1, the element can be replaced with a circuit configuration composed of a capacitive component C and a component E of a diode characteristic coupled in parallel with the capacitive component C. Thus, the EL element can be regarded as a capacitive light-emitting element. As the EL element is applied with a direct current light-emission driving voltage across the electrodes, a charge is accumulated in the capacitive element C. Subsequently, when the applied voltage exceeds a barrier voltage or a light emission threshold voltage inherent to the element, a current begins flowing from one electrode (on the anode side of the diode component E) to the organic functional layer which is a light emitting layer so that light is emitted therefrom at an intensity proportional to this current.
The Voltage V—Current I—Luminance L characteristic of the element is similar to the characteristic of a diode, as illustrated in FIG. 2. Specifically, the current I is extremely small at a light emission threshold voltage Vth or lower, and abruptly increases as the voltage increases to the light emission threshold voltage Vth or higher. The current is substantially proportional to the luminance L. Such an element, when applied with a driving voltage exceeding the light emission threshold voltage Vth, exhibits a light emission luminance in proportion to a current corresponding to the applied driving voltage. On the other hand, the light emission luminance remains equal to zero when the driving voltage applied to the element is at the light emission threshold voltage Vth or lower which does not cause the driving current to flow into the light emitting layer.
As a method of driving a display panel using a plurality of EL elements as described above, a simple matrix driving mode is known. FIG. 3 illustrates an exemplary structure of a driver in accordance with the simple matrix driving mode. In a light emitting panel, n cathode lines (metal electrodes) B1–Bn are arranged extending in parallel in the horizontal direction, and m anode lines (transparent electrodes) A1–Am are arranged extending in parallel in the vertical direction. At each of intersections of the cathode lines and the anode lines (a total of n×m locations), an EL element E1,1–Em,n is formed. The elements E1,1–Em,n which carry pixels are arranged in matrix, each have one end connected to an anode line (on the anode line side of the diode component E in the aforementioned equivalent circuit) and the other end connected to a cathode line (on the cathode line side of the diode component E in the aforementioned equivalent circuit) corresponding to the intersections of the anode lines A1–Am along the vertical direction and the cathode lines B1–Bn along the horizontal direction. The cathode lines are connected to a cathode line scanning circuit 1, while the anode lines are connected to an anode line drive circuit 2.
The cathode line scanning circuit 1 has scanning switches 51–5n corresponding to the cathode lines B1–Bn for individually determining potentials thereon. Each of the scanning switches 51–5n supplies a corresponding cathode line either with a bias potential Vcc (for example, 20 volts) or with a ground potential (zero volt).
The anode line drive circuit 2 has current sources 21–2m (for example, regulated current sources) corresponding to the anode lines A1–Am for individually supplying the EL elements with driving currents through respective anode lines, and drive switches 61–6nEach of the drive switches 61–6n is adapted to supply an associated anode line with the output of the current source 21–2m or a ground potential. The current sources 21–2m supply the associated elements with such amounts of currents that are required to maintain the respective EL elements to emit light at desired instantaneous luminance (hereinafter this state is called the “steady light emitting state”). Also, When an EL element is in the steady light emitting state, the aforementioned capacitive component C of the EL element is charged with a charge, so that the voltage across both terminals of the EL element is at a positive value VF (hereinafter, this value is called the “forward voltage”) slightly higher than a light emitting threshold voltage Vth. It should be noted that when voltage sources are used as driving sources, their driving voltages are set to be equal to VF.
The cathode line scanning circuit 1 and the anode line drive circuit 2 are connected to a light emission control circuit 4.
The light emission control circuit 4 controls the cathode line scanning circuit 1 and the anode line drive circuit 2 in accordance to the image data supplied from an image data generating system, not shown, so as to display an image represented by the image data. The light emission control circuit 4 generates a scanning line selection control signal for controlling the cathode line scanning circuit 1 to switch the scanning switch 51–5n such that any of the cathode lines corresponding to a horizontal scanning period of the image data is selected and set at the ground potential, and the remaining cathode lines are applied with the bias potential Vcc. The bias potential Vcc is applied by regulated voltage sources connected to cathode lines in order to prevent crosstalk light emission from occurring in EL elements connected to intersections of a driven anode line and cathode lines which are not selected for scanning. The bias potential Vcc is typically set equal to the light emission regulating voltage VF (Vcc=VF). As the scanning switches 51–5n are sequentially switched to the ground potential in each horizontal scanning period, a cathode line set at the ground potential functions as a scanning line which enables the EL elements connected thereto to emit light.
The anode line drive circuit 2 conducts a light emission control for the scanning lines as mentioned above. The light emission control circuit 4 generates a drive control signal (driving pulse) in accordance with pixel information indicated by image data to instruct which of EL elements connected to associated scanning lines are driven to emit light at which timing and for approximately how long, and supplies the drive control signal to the anode line drive circuit 2. The anode line drive circuit 2, responsive to this drive control signal, individually controls the switching of the drive switches 61–6m to supply driving currents to associated EL elements through the anode lines A1–Am in accordance with the pixel information. In this way, the EL elements supplied with the driving currents are forced to emit light in accordance with the pixel information.
Next, the light emitting operation will be explaining with reference to an example illustrated in FIGS. 3 and 4. This light emitting operation is taken as an example in which a cathode line B1 is scanned to have EL elements E1,1 and E2,1 emit light, and subsequently, a cathode line B2 is scanned to have EL elements E2,2 and E3,2 emit light. Also, for facilitating the understanding of the explanation, in FIGS. 3 and 4, an EL element which is emitting light is represented by a diode symbol, while an element which is not emitting light is represented by a capacitor symbol.
Referring first to FIG. 3, only a scanning switch 51 is switched to the ground potential equal to zero volt to scan a cathode line B1. The remaining cathode lines B2–Bn are applied with the bias potential Vcc through the scanning switches 52–5n Simultaneously, anode lines A1 and A2 are connected to current sources 21 and 22 through drive switches 61 and 62, respectively. The remaining anode lines A3–Am are switched to the ground potential equal to zero volt through drive switch 63–6m Thus, in this event, only the EL elements E1,1 and E2,1 are forward biased so that driving currents flow thereinto from the current sources 21 and 22 as indicated by arrows, causing only the EL elements E1,1 and E2,1 to emit light. In this state, the EL elements E3,2 and Em,n which are not emitting light, indicated by hatching, are charged with polarities as indicated in the drawing.
From the light emitting state illustrated in FIG. 3, only the scanning switch 52 corresponding to the cathode line B2 is now switched to the ground potential equal to zero volt to scan the cathode line B2 as illustrated in FIG. 4. Simultaneously with this scanning, the current sources 22, 23 are connected to the corresponding anode lines A2, A3 through the drive switches 62, 63 while the remaining anode lines A1, A4–Am are applied with zero volt through the drive switches 61, 64–6m respectively. Thus, in this event, only the EL elements E2,2, E3,2 are forward biased, so that driving currents flow into the EL elements E2,2, E3,2 from the current sources 22, 23 as indicated by arrows, causing only the EL elements E2,2, E3,2 to emit light.
As described above, the light emitting control is made up of repetitions of a scanning mode that is a period in which any of the cathode lines B1–Bn is activated. The scanning mode is performed every one horizontal scanning period (1H) of image data, wherein the scanning switches 51–5n are sequentially switched to the ground potential every horizontal scanning period. The light emission control circuit 4 generates a driving control signal (driving pulse) in accordance with pixel information indicated by image data to instruct which of EL elements connected to associated scanning lines are driven to emit light at which timing and for approximately how long, and supplies the drive control signal to the anode line drive circuit 2. The anode line drive circuit 2, responsive to this drive control signal, controls the switching of the drive switches 61–6m to supply driving currents to associated EL elements through the anode lines A1–Am in accordance with the pixel information. In this way, the EL elements supplied with the driving currents are forced to emit light in accordance with the pixel information.
There is a driver which is capable of displaying in gradation for representing the contrast of an image on the display panel using EL elements as described above. PWM (Pulse Width Modulation) is typically employed for gradation display. Specifically, the driver generates a pulse having a width in accordance with a specified gradation level determined by pixel information in a constant one-horizontal scanning period to activate a current source only for the duration of the pulse width to supply a driving current to EL elements to be lit. During the remaining period of the one-horizontal scanning period, the driver inactivates the current source to stop supplying the driving current from the current source.
However, the driver for conducting a gradation display has a problem of deteriorated linearity in the gradation display due to the fact that a current generated by the bias potential Vcc flows into EL elements through other EL elements on the same anode line to prevent the light emission from immediately stopping immediately after a transition from an active state from an inactive state of the current source within one horizontal scanning period.
Specifically, explaining one horizontal scanning period in which an EL element E1,1 is driven to emit light from among EL element E1,1–E1,n connected to an anode line A1 of the driver illustrated in FIGS. 3 and 4, a driving current from a current source 21 flows into the ground through a drive switch 61, anode line A1, EL element E1,1, cathode line B1, and scanning switch 51 during an activated period of the current source 21, causing the EL element E1,1 to emit light, as illustrated in FIG. 5. In this event, the remaining EL elements E1,2–E1,n connected to the anode line A1 are applied with a substantially equal potential at both ends thereof, so that no current flows into the EL elements E1,2–E1,n. For example, when the bias potential Vcc is set at 20 V, the potential on the anode line A1 is 20 V, so that the lighting EL element E1,1 is applied with 20 V in the forward direction. At the time the current source 21 transitions from the active state to the inactive state, the EL element E1,1 is discharged through light emission, resulting in a reduction in the voltage on the anode line A1, as illustrated in FIG. 6. With the reduced voltage on the anode line A1, a charging current to the EL elements E1,2–E1,n is driven by the bias potential Vcc to flow into the ground through each of the EL elements E1,2–E1,n anode line A1, EL element E1,1 cathode line B1 and scanning switch 51 Thus, as illustrated in FIG. 6, the EL element E1,1 is applied with a voltage higher than the light emission threshold voltage Vth in the forward direction, so that the EL element E1,1 continues to emit light. On the other hand, since each of the EL elements E1,2–E1,n is charged with a charging current of opposite polarity, the charging current level becomes lower as they are charged more. The voltage applied to the EL element E1,1 in the forward direction, i.e., the potential on the anode line A1 is also reduced with the passage of time as illustrated in FIG. 7, so that the light emission luminance of the EL element E1,1 becomes gradually lower, and eventually, the light emission is stopped.
As a result, a linear relationship is not established between the pulse width generated corresponding to a specified gradation level and the brightness provided by light emitted by the EL element. Specifically, when a narrow pulse width is generated corresponding to a specified gradation level, actual light emission will result in an excessively bright display, failing to provide the brightness corresponding to the pulse width.